1. Field of the Invention
The present invention relates to the field of kaolin and kaolin products which are particularly useful for use in both paper coating and paper filling. More particularly, the present invention relates to kaolin clay products having unique coarse platey and high brightness characteristics and to methods for making and using such coarse platey, high brightness kaolin products.
2. Background
Fillers are utilized in the paper industry to reduce the amount of wood pulp needed to manufacture paper and provide optical and printing properties to the paper being produced. They are also used in other products including paints and plastics. Among the fillers suitable for use as fillers or coatings are titanium dioxide, calcium carbonate, silica, alumina and kaolin. Of these, kaolin, a hydrated aluminum silicate, is the most widely used.
Kaolin is obtained from kaolin crudes. The crudes can contain kaolin particles, oversize particle size impurities (grit), and fine and coarse particle size impurities, such as fine ferruginous or titaniferous impurities, which impart undesirable color to the clay. Kaolin particles typically occur in the crude over a wide range of particle sizes and shapes. Typically a kaolin crude, such as a typical Georgia crude will contain, after removing the grit, particles ranging in size from submicron or colloidal to 20 microns or larger. Moreover, the kaolin morphology typically includes arrangement in plates, which plates can be further arrayed in stacks. Particle size, impurity content and morphology can vary with location of the deposit and within any given deposit itself.
Kaolin particle size is typically determined by sedimentation using Stokes law to convert settling rates to particle size distribution and assuming a spherical shape of the particles. Particle size determined by this method is measured in terms of equivalent spherical diameter, or e.s.d., as determined by Sedigraph.
Kaolin brightness is typically determined by on a GE scale, where GE is measured as a percent brightness relative to magnesium oxide (MgO). As illustrated in Table I, the GE brightness of fine particle kaolins such as North American delaminated kaolin is 90. The GE brightness of coarse particle kaolins such as coarse European kaolins or coarse North American kaolins is 81-85, respectively.
Historically, kaolin pigments are available in a wide variety of grades and the selection of the particular grade of kaolin pigment used in a paper was as much a function of price as it was a function of its end use. Where the end use of the kaolin is as a filler or pulp extender to fill paper, the emphasis has typically been on the use of coarse particle kaolins such those that are disclosed and claimed, for example, in U.S. Pat. No. 4,943,324 (Bundy, et al.). Where the end use of the kaolin was as a coating to provide the paper with a smooth, aesthetically appealing appearance, however, the focus has typically been on the use of fine particle kaolins which have higher brightness and opacity characteristics such as those that are disclosed and claimed, for example, in U.S. Pat. Nos. 5,645,635 (Behl, et al.) and U.S. Pat. No. 5,169,443 (Willis, et al.).
The present invention provides a novel kaolin product and methods for making a novel kaolin product not heretofore known in the industry. The kaolin products of the present invention combines the unique characteristics of large particle size only found in coarse particle kaolins with high particle brightness characteristics only found in fine particle kaolins. The kaolin products of the present invention also demonstrates the unique characteristic of having a high shear rheology which enables the spraying drying of the product. The kaolin product of the present invention adaptable for use both as a paper filler and a paper coating. The methods of the present invention enable production of a number of coarse platey, high brightness kaolin products, each having different coarse platey and high brightness characteristics. The methods of the present invention also enable the preservation of resources by utilizing kaolin fractions which may have been previously discarded when fabricating one or the other of a coarse or fine kaolin product.
The present invention provides at least one coarse platey, high brightness kaolin product and a method for making this kaolin product. The kaolin product produced by the method of the present invention has the unique qualities of high brightness, good opacity, very good high and low shear viscosity and large particle size, which make them equally adaptable for coating and filler applications, as well.
Specifically, the present invention provides coarse platey, high brightness kaolin products which are formed by the combination of a coarse kaolin fraction and a fine kaolin fraction. Each coarse platey, high brightness kaolin product produced by this method has a particle size of at least 40% greater than two microns (xcexc) and a minimum high shear rheology of 800 rpm at 18 dynes and 68% solids using A-bob on a Hercules Viscometer. As is preferred, the coarse platey, high brightness kaolin product of the present invention may be spray dried or combined with a suspending agent to stabilize the kaolin slurry product for use, storage and/or shipment.
The coarse and fine kaolin fractions of the kaolin products of the present invention consist of one or more crude kaolin samples which are processed through different stages of the methods of the present invention, of a blending of one or more crude kaolin samples so processed, or of a by-product of the processing of a crude kaolin sample to a coarse or fine kaolin product.
The methods for making a coarse platey, high brightness kaolin products include the steps of combining a coarse kaolin fraction with a fine kaolin fraction to form the coarse platey, high brightness kaolin products. Depending on the desired manner of use, shipping and/or storage, the methods of the present invention further include the step of either spray drying or adding a suspending agent to the slurry to stabilize the coarse platey, high brightness kaolin product after combination of the two kaolin fractions.
The coarse platey, high brightness kaolin products of the present invention are produced by varying the quantity of each coarse and fine kaolin fraction which is combined in accordance with the methods of the present invention. The variability of the quantity of coarse and fine kaolin fractions combined to form the kaolin product of the present invention and the possible different combinations of coarse and fine kaolin fractions enable the production of a number of kaolin products having a unique coarse platey and high brightness characteristics.